docs: def add(self, attribute):
          """Add an attribute to this attribute exchange request.
  
          @param attribute: The attribute that is being requested
          @type attribute: C{L{AttrInfo}}
  
          @returns: None
  
          @raise KeyError: when the requested attribute is already
              present in this fetch request.
          """
          if attribute.type_uri in self.requested_attributes:
              raise KeyError('The attribute %r has already been requested'
                             % (attribute.type_uri,))
  
          self.requested_attributes[attribute.type_uri] = attribute